Placenta and Fetal Membranes Formation

Placenta and Fetal Membranes Formation

• Fertilization occurs in the ampulla of the fallopian tube within 12 hours of ovulation (takes 24 hours).
• Sperm passes through corona radiata and zona pellucida, reaching the ovum membrane.
• Zona pellucida prevents further sperm entry.
• Ovum completes second maturity division (female pronucleus).
• Sperm tail degenerates, the head forms the male pronucleus.
• Male pronucleus fuses with the female pronucleus, creating a zygote with 46 chromosomes.

Zygote Development

• Zygote divides, forming 2 cells after 30 hours.
• Zygote forms 16 cells (morula) 3 days after fertilization.
• Blastocyst forms, containing a fluid-filled space.
• Inner cell mass develops, forming the embryo and amnion.
• Trophoblast forms the placenta and chorion.

Implantation

• Morula enters the uterine cavity on day 3.
• Endometrium develops, enriched with glands and blood vessels.
• Blastocyst implants in the uterine body (anterior or posterior).
• Blastocyst attaches to the endometrium around days 7-8.
• Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast.
• Blastocyst penetrates the endometrium, with implantation taking 3 days, and entry site closes.
• Implantation completes around the 10th or 11th postovulatory day.

Decidua

• When implantation is complete, the endometrium becomes the decidua.
• Progesterone causes endometrial stromal cells to change into decidua cells (large, specialized cells).
• Decidua contains blood vessels and glands.
• Glands and blood vessels in the decidua regress.

Decidua Types and Functions

• Decidua basalis: Under the embryonic plate.
• Decidua capsularis: Covers the blastocyst.
• Decidua parietalis: Covers the rest of the uterus.
• Decidua functions to prevent maternal tissue invasion by fetal tissues, provides fetal nutrition, and produces hormones (prostaglandins and prolactin).

Chorionic Villi Formation

• Cytotrophoblast and syncytiotrophoblast proliferate to form chorionic villi, which spread out branching throughout the decidua space.
• Syncytiotrophoblasts degenerate neighboring tissues, the decidua.
• Maternal blood vessels are lacerated in the decidua, forming lacunae (spaces).
• Chorionic villi degenerate except near myometrium.
• This creates the chorion frondosum (placenta) and chorion leave.

Chorionic Villus Types

• Primary villi consist of cytotrophoblasts and syncytiotrophoblasts.
• Secondary villi form when mesenchymal cells from the extraembryonic mesoderm invade the primary villi.
• Tertiary villi form when blood vessels develop within the secondary villi.
• Intervillous spaces are the spaces between chorionic villi.

Placenta Formation Summary

• Placenta originates from the chorion frondosum: a set of chorionic villi.
• Chorionic villi originate from the chorionic plate and grow in a lake of maternal blood within intervillous spaces.
• Placental lobules develop from secondary stem villi.
• Each lobule has its own, single utero-placental artery.
• Placental bed forms from the maternal side (fetal trophoblast and decidua basalis).

Placenta Characteristics

• Diameter around 22 cm.
• Weight around 500g.
• Fetal side is covered by amnion; chorionic vessels continue to the umbilical cord in a star-like pattern.
• Placental plate on the fetal side.
• Basal plate on the maternal side.
• Chorionic + basal fusion creates a chorionic membrane.

Placenta Functions

• Respiratory: Oxygen and carbon dioxide exchange via simple diffusion.
• Nutrient: Glucose (facilitated diffusion), amino acids (active transport), fatty acids (simple diffusion), lipoproteins, and steroids pass; proteins are relatively impermeable except IgG (immunoglobulin G).
• Endocrine: Produces hormones, such as estrogen, progesterone, prolactin, human placental lactogen, and human chorionic gonadotropin.

Inner Cell Mass Development

• Rapid proliferation of inner cell mass cells begins around day 10 after fertilization.
• Blastocyst cavity changes into chorionic cavity lined by chorion.
• Two cavities form around the embryonic plate.

Fetal Membranes (Amnion and Yolk Sac) Development

• Amnion (amniotic sac) forms adjacent to the embryonic ectoderm, with its basal layer and roof developing from cytotrophoblast.
• Yolk sac forms adjacent to embryonic endoderm; it originates from the endoderm layer.
• Two cavities—separated by the embryonic plate—differentiate into ectoderm, endoderm, and mesoderm.

Additional Summary Points

• The chorionic blood vessels form the umbilical vein and arteries (2 arteries and 1 vein).
• The umbilical vein carries oxygenated blood to the fetus, whereas the umbilical arteries carry deoxygenated blood from the fetus.
• Anchoring villi reach the basal plate, and the distal part of the villus stem is formed by cytotrophoblast, which spreads to form the trophoblastic shell.
• Syncytiotrophoblast growth/maintenance depends on cytotrophoblast.

This quiz covers the intricate processes involved in the formation of the placenta and fetal membranes, starting from fertilization to implantation. It details the development stages of the zygote, including the morula and blastocyst formation, as well as the implantation process within the uterine cavity.